Equalizer for transmission lines



April 24, 1951 K. W. PFLEG ER EQUALIZER FOR TRANSMISSION LINES OriginalFiled Nov. 18, 1947 5 Sheets-Sheet 1 smumo Z TRANSMITTING sumo/vrnzousncr l2 l OSCILLATOR l4 I 30 1451555 5: wan/p 3 .9010 PASS HARMONICsm r050. 60/!- FILTER GENERATOR I w gg s a v NARROW BAND mss FILTERSfifi fi map I {570. FREQ- smell. kw uaoummn fl/a cmcu/rs I T TELEVISION4/ i SEND/N6 5 smu. ssr 512 A; %;0

VIDEO SIGNAL MODULATED WAVE CON TA "IN/N6 VIDEO SIGNAL rou. LINE 1 my UPHASE AND LEVEL MANUAL ADJUSTERS FREQUENCIES INVENTOR K. W PFLE GE R A7T ORNE V April 24, 1951 K. w. PFLEGER 2,550,596

EQUALIZER FOR TRANSMISSION LINES Original Filed Nov. 18, 1947 5Sheets,Sheet 2 FIG? RECEIVING STAT/ON 'E To "Z (en-a)! LESS 1v mar mmBACK CIRCUIT ATTENUAT/ON EQUAL/25R 1 (F165 0R F163) THERM/Smfi HEATERSCONTROL CONTROL CIRCUITS FOR LOSS N FHA 55 AND LEVEL 40.1.

(FIG. .3)

BAND PASS FILTERS lNl/E/VTOR K. W PF LE GER ATTORNEY 5 Sheets-Sheet 4l+f2/V- FIXEZ HUMP RESUL 1.4 1v '1- +(2/va) F (SEE rm. 5)

FIG. 7

T0 THERM/STORS IN 52 K. w. PFLEGER EQUALIZER FOR TRANSMISSION LINES FIG.6'

I V ADJUSTABLE HUMPS FIXED HUMP April 24, 1951 Original Filed Nov. 18,1947 U m an uvvlewron K W'PFLEGER N PILOT FREQUENCIES IN BRANCH C 0FFIG.2

F m 2 r m F 3 4 a u 2 l 5 F m 6 0 Z 2 n w 2 9 0 0 w .m 0 4 I 2 m m c m ms E w o m 5 s a M W. P I- a m 5 LOW PASS FILTERS DEMODULA TORS N-ICHANNELS 0F FREQ 2F-- Patented Apr. 24, i951 EQUALIZER FORTRAN SMISSIONLINES Kenneth W. Pfleger, ArlingtonyNrJg assignor to Bell TelephoneLaboratories, Incorporated, N ew -York, N. Y., a corporation of New YorkOriginal application November 18, 1947,..Serial No. 786,745. Dividedandthis applicationJuly 19,1949,SerialNo.105,518 V r 4 Claims. (chm-#44)This invention relates to electric circuits and more particularly tocircuits for equalization of waves transmitted over transmission linesor other media. This application is a division of application Serial No.786,745, filed November 18, 1947.

. It is an object of this invention to improve the equalization of widebandsignals and especially of those signals transmitted over a pathwhich is i so long electrically as to cause phase shifts betweencomponents of different frequencies.

The transmission of television signals-involves many problems inconsequence of the relatively wide band'of frequencies over which thesignals are spread. Not the least of these problems is that ofequalizing or compensating for phase variations caused, for example, bytemperature changes entransmission line circuits carrying these widehand signals. It has previously been proposed to provide basiccompensation for such phase variations by the use of equalizing networkswhich introduce fixed envelope delay (dB/(1w) into the system and tofurther compensate for these variations by providing adjust able delayequalizers in conjunction with the fixed of stationary impedanceelements. The impedance of each of the adjustable elements is changed bymeans of a current of varying intensity caused to be varied'inaccordance with the phase difference of two waves of the same frequency(F0) obtained respectively by demodu lating two modulated wavestransmitted over the television line, .each comprising a pilot frequency(a different one for each one of the waves) modulated by the fixedfrequency Fo which is. lower than either of the pilot frequencies. Thetwo demodulated waves of the frequency F0 are applied to apush-pull'phase detector after having been initially adjusted by phaseshifters to be in quadrature. As long as this phase relation ismaintained,the output of the phase devif 2 r tector is zero,regardlessof level changes on the television line. When the phaserelation between the two demodulated waves changes due to variationsindelay over the line, the balance is destroyed and the output currentof the phase detector has an appreciable magnitude in one direction orthe other. When the phase relation between the input currents for thephasedetector changes in the opposite direction, the direction ofOutputcurrent is reversed. These output currents are, as pointed out above,utilized to control the impedances of variable elements in an adjustableequalizer. A first pilot frequency F1 is located at or slightly belowthe lower edge of the television band, asecond pilot frequency F2 islocatedabout in the center of the band (preferably in a dead portion ofthe television band), and athird at or slightly above the upper edge ofthe television band. The delay distortion over the frequency range F1 toF2 is corrected with the aid of one of the phase detectors while thedistortion over the frequency range F2 to F3 is corrected with the aidof the second phase detector. In the system described in theabovementi'oned' Pfieger patent; the sidebands are transmitted over thecable or other path and utilized with the pilot frequency waves at thereceiving endto produce waves at the envelope frequency which waves areapplied to the phase detectors.

In a mop-up" equalizer utilizing an adjustable attenuator or equalizerin accordance with the present invention (the mop-up equalizer in one ofits aspects being a special case ofand an extension of the patentedPfieger arrangement) both automatic delay distortion and automaticattenuation distortion are compensated by the same pilot frequencies, ofwhich a great number are provided, they preferably being locatedthroughout the television band at intervals corresponding to the linescanning frequency of the television image. spacing'of the pilotfrequencies, only the latter need be transmitted and the step ofmodulation.

(inthe above-identified patented arrangement) is not, ;necessary. Thepilot frequencies are comprising a multiplicity of parallel-connectedDue to the relatively close a multiplicity (for example there might be191 for television having 441 lines and 30 frames per second) of narrowband-pass filters 20, 2!, 22, 23, 24, 25 29, 30 tuned to frequenciesseparated by a frequency of 13.23 kilocycles (the line scanningfrequency), a multiplicity of frequencies are selected as follows: Fe(carrier frequencyfor example, 300 kilocycles per second), FcF (where Fis one-half line scanning frequency), Fc-i-F,

in connection with the accompanying drawings forming apart thereof inwhich:

Fig. 1 is a schematic block diagram of a portion of the sending endapparatus of a television system including the sending end'equipment ofa mop-up equalizer embodying an adjustable attenuator or equalizer inaccordance with the invention;

F is e ch mati b eep diagra ef ee iviiie end ap aratu adapted-to be uized it he. sendin end qu ment sho in i i;

i Bis. a circ it dia ra of o f t m nua level and ph s ad te s fo ming,pa t i. h equipm nt show Fi l and Fi Bi i a. circuit dia ram of. one othe co t o circu ts. formi per of. the arrangemen of i r Fi 5 is. aci idia ram o roup of typical disto t on equa ze s su tab e or u e. in the:rran eme t of F g- 2;

Fig.- 6 is a raphical e sentat on o a d underst ndin he i v n n; I

Fig, '7 is a schematic block diagram of control circuits which can be utlized inthe a n ement of Fig- 2;

Fi 8 isa circuit, dia ramo .a pha e d tector which can be used in the,control circuits: of Fig. 7;

J Fig. .9; is a schematic block diagram of an at-. tenuation equalizerin accordance with the invention which produces practically no delaydistortion; and

Fig. 1Ov is agraphical representation to aid in understanding theoperation :of, the arran ement of'Fig. 9; r

- Referring-more. specifically to the drawings, Fig.8 shows an exemplaryembodiment of an attenuation equalizer in accordance with the in.-vention. In order to illustrate the mannen. use and advantages. of. suchan. equalizer, however, reference-will first be made to Figs. 1 and 2which show, in block diagram form, sending and'receivingend portions l0and I I, respectively, of: a telee. vision system in which mop-upequalization of loss and envelope delay are employed; For sim.-.-plicity in the drawings, details of the equipment for generating,transmitting and utilizing the video signal at the receiving. station toproduce an image of the object have not been shown since thepresentinvention is concerned primarily with the los and delayequalization of the television signals.

- Referring first to the sending end equipment Ill shown in Fig. 1, astandard frequency oscillator 12 generates oscillations of, for example,1000 cycles. per second, and this generated wave is applied through ahybrid coil I3- to a multivibrator and harmonic generator M- of anysuitable form to produce a group of accurate frequency waves. (Thestandard frequency wave can also be applied through the filter 30 to thesynchronizing-circuits forming part of the. television sending: set l5,for control purposes.)- By means of Fc--3F, Fc+5F, Fc+(2N-3)F. Where thelower sideband of the television signal is suppressed, it is-necessar-yto transmit relatively few pilot frequencies below Fe. The carrierfrequency Fe is modulated inthe modulator 16 with a video signal passingthe hybrid coil l3 from the television sending set 15. and the resultantmodulated Wave is passed through a single sideband filter I1 and theamplifier 18 to the toll line I9.

The selected frequencies FcF up to Fc+(2N3)F are each passed through anindividual one of the menuallyiu tedp e e a d level ad us e s 3 32, 33,34, 35 39, 40 (to compensate for any phase or level change in thefilters). Suitable phase and level adiusters are shown in Fig. 3 andwill be described below; Each of the selected frequencies is then passedthrough an ini i ue e o he narr w ba dfi ters 4213,44, 49,.5lltopreventthat frequency from int ractin w t the phase or level adjus r or any ofthe o her. fre uen es- Th nan or h ted quencies are ap ied hr h theamplifier 18 to the toll line 19 and transmitted to the receivingstation I I shown in Fig. 2.

At the receiving station I-l of Fig. 2, all of the selected (pilot)frequencies (Fc-F to Fe+(2N.3)F, inclusive) as well as the transmittedmodulated wave containing the video si n ls e epe se thr ush an ampifier 5| and then through. a. delay distortion equalizer 52 (which will.bedescribed more fully below in connection with Fig. 5 or Fig. 9) andanother amplifier 53. The output current of the amplifier 53 is dividedinto three parts as follows: (1) a poron going throu h a res stance pad54 to an output ircu te minal 55 to wh h r applied (by means to bedescribed below) the. pilot frequenei s o oppose those in the am lifier53 and thus l ave only the, transmitted; modulated wave containing the.video signal, (2) a portion going th ou h a. feedback circuit containingvariable at nua on equaliz rs 55 (see Fig. 5: or Fig. 9). and. (3) aportion going through a multiplicity of parallel circuits. eachcomprising one of the narrow band-pass filters GI, 62, 63, 64, 65 69, loand one of the amplifiers ll, l2, l3, 14, 1.5, 19, 8B. The output.current from each. of the last-mentioned ampl fiers. is .then' dividedinto three parts, (1) a portion. (A) going through an individual oneofthe control circuits. 5'! for the loss adjuster (which circuits willbe described more fully below in connection with Fig. 4) which varies,the current. through an individual one of the thermistor heatersBl;,.82, 83, 84, 89, 9.0 in one of the. attenuation equalizers of thefeedback circuit 56, (2). a portion (B) going through. individual phaseand. level adjusters 58 (which will be described more fully below inconnection withFig. 3) and narrow band-pass filters 91, 92 93', 94, 95-9.9, lllllto the output circuit terminal 55; whereby the pilotfrequencies are adjusted in phase and amplitude to oppose thosein'theoutput circuit of amplifier 53, and (3) a portion C') going to-anindividual one of the control: circuits 59 (which will be. describedmore fully below in connection with Fig. 'D-for varying for anindividual impedance member of the distortion equalizer 52 shown in Fig.5 or Fig. 9.

.Referring now to Fig. 5, this figure shows one form of equalizer whichmay be used as the delay distortion equalizer 52 or the attenuationequalizer 56. It comprises a first section IOI consisting of amultiplicity of parallel-connected branches III, H3, H5, II! and H9, Bnrespectively resonating at a. different one of the odd-numbered pilotfrequencies and a thermistor, followed after resistance pad I02 by asection I03 like thefirst section IOI but tuned at the evennumberedpilot frequencies. Considering one of said branches, as, for example,the branch III, it comprises a capacitor member I04, an inductancemember I 05, resistance member I06 and a thermistor member I07, theother parallel branches H3, H5, H1, H9, Bn, and also those in theeven-numbered section I03 are simic lar to the branch III except thatthe frequency of the tuned circuit varies. By varying the resistance ofa particular thermistor I6! by varying the current in its correspondingheater 8|, 83, 85, 81, 89, Hn, the magnitude of the hump of lossinserted into the system at that pilot frequency can be varied. Theresultant of the two sections IIII and I03 is substantially fiat for theover-all frequency range. A the arrangement shown in Fig. 5 producesboth attenuation and therewith a similar circuit I03 having the parallelbranches thereof tuned to even-numbered pilot frequencies. At averagevalues of the thermistor, the loss hump has about the same peak value atall pilot frequencies and the loss characteristic due to the one networkis complementary to that of the other so that over-all transmissionis-fiat when all the thermistors are at an average value of resistance.The use of sinusoidal humps is advantageous. In general, for averagevalues of thermistors or other possible variables, the odd-numberednetwork IOI produces an over-all loss, for example, as shown by the dashline curve in Fig. 6. The even-numbered network I03 gives thecomplementary loss shown by the full line curve so that the resultant isfiat for the frequency range of interest. By variations from the averageof one or more networks, the resultant can be made to have eithernarrow, or broad humps depending on the changes occurring in adjacentchannels. On Fig. 6, which is a plot of loss versus frequency, the losscurves for both halves of the equalizer and the resultant have beenshifted to refer them to an arbitrary zero loss axis in order tofacilitate comparison. A curve similar to Fig. 6 can be drawn for delayversus frequency.

. When simple networks of the type shown in Fig. 5 are used for lossequalizers, delay humps are also produced and when they areused fordelay equalizers, loss humps are also produced. Resonant shunts areshown in Fig. 5 merely to illustrate how the pilot channels can controlhumps of loss or delay. In practice, some other form of tuned networkmight be preferable. As an example of an attenuation. equalizer whichproduces practically no delay distortion, reference will now be made toFig. 9.

In the arrangement of Fig. 9, there is shown an attenuation equalizer inaccordance with the invention which produces substantially no delaydistortion. The equalizer of Fig. 9 comprises a plurality ofparallel-connected branches I2I, I22, I23, I24, I25, I26 I30 eachconsisting of a respective band-pass filter I3I, I32, I33, I34, I35, I36or I40 tuned to a different pilot frequency and adapted to pass a nearlysinusoidal p quency, a respective fixed delay equalizer I4 I, I 42, I43,I44, I45, I46 or I50 for compensating for any delay in the correspondingfilter and a respective variable resistor element I5I, I52, I53, I54,I55, I56 or I60 (which maybe thermistors) for varying the intensity ofthe hump." The equalizer of Fig. 9 can also be in two sections like theone of Fig. 5, one section having the oddnumbered branches and the othersection having the even-numbered branches. By means of this arrangement,humps of correct amount of loss can be inserted into the system at theproper frequency to give substantially fiat compensation.

The characteristics of the nth channel. are raphically shown in Fig. 10,the characteristics of the various channels being similar. The currentmagnitude when plotted versus frequency has a maximum at Fn, the nthpilot frequency, and is preferably negligible at and beyond frequencyFn+1 and Fn-l respectively corresponding to the pilot channels n+1 orn-1. The delay of the filter alone has humps as shown by the dash linecurve of Fig. 10. It is for this reason that delay equalizers areincluded in each channel of the arrangement of Fig. 9 to make the delayconstant over the transmitted frequency range of the channel. The totaldelay is the same I for each channel in its transmitted frequency range.The resistance (I5I I60) in the output of each of the parallel-connectedbranches of the circuit of Fig. 10 can be varied. It is assumed that allimpedances as seen from the resistances I5I I60 have been adjusted tohave zero angle which can be done by using constant K- type networks.Therefore these resistances I5I I60 control the relative outputs of thechannels without altering the delay or the shape of the amplitude versusfrequency characteristics. When the resistances are set so that eachchannel transmits the same current magnitude at its mid-band frequency,and when the amplitude versus frequency characteristic of each channelabove its mid-band frequency is complementary to that of the next higherchannel below its midband frequency, then the over-all transmissionversus frequency characteristic for the entire circuit of Fig. 9 will beflat from F1 to the mid-band frequency of the highest channel. Gain 01'loss humps can next be inserted at will by varying the adjustableresistors I5I I60 with no effect on the delay since all channels havethe same constant delay. Where the numbers of channels in parallel isgreat there will be considerable lossdue to the shunting effects of somany branches at each junction point. This may be reduced if necessaryby the use of amplification and the subdivision of channels into groups,each with a separate amplifier if necessary. The resistors I5I I60 canbe thermistors operated by heaters similar to those in Fig. 5, ifdesired.

A combination of an equalizer of Fig. 9 with an equalizer which producesboth delay and attenua symmetrical around the tuned freassures:

7' tion effects or delay eirects only permits.- compen sation of anydesired delay'or attenuation distorti'ons; either separately ortogether;

In the circuitof Fig. 2f the: delay distortion equalizer: 5.2 (such: asthat. shown in Fig. 5 or Fig. 9) is shown at the input. to. theamplifier 5-3. in order. to prevent unfavorable: phase. relations fromarising. in the feedback circuit including. the attenuation equalizer56-. It is intended that small incidental lose variations caused by the.delay network be. automatically compensated by the: attenuation.equalizer and incidental. delay variations of the latter, ifnot. too:great be compensated f'or' b the. delay equalizers. By providingpractically instantaneous controlsusing fastacting thermistors in. thevariable networks, the. two; types of compensation can be made tocooperate. simultaneously in producing over-all flat loss and; delayversus. frequency" characteristics.

a Each of the. control. circuit 51. in branch. A. of the circuit. ofFig. Zcan be of any suitable form. An; example of one satisfactorycircuit. is shown in Fig 4.. In the arrangement of Fig, each. pilotfrequency is: applied to a rectifier I61 through a high frequencyattenuator I62: having. constant impedance. The output of the rectifier'appearing: across the resistor IE3 is applied to any convenient form of.oscillator I64 to control. the. variable output to heat any one ofthe-thermistorsa- 81, 82,. 83 910. The connections in Fig. 4 should bepoled so that a sudden increase in levelof a pilot channel on the linecauses the corresponding thermistor resistance to vary in a direction toincrease the amount. of negative feedback (through the attenuationequalizer 5'6) permitted at this frequency, until the level of the pilotfrequency" at the amplifier output. is reduced to: the. former value. A.similar procedure. follows with opposite signs when the ilot level.decreases.

The branch B of circuit shown inv Fig; 2. includes a. multiplicity ofphase: and level adjusters: 58. A suitable manual level and phaseadjuster is shown in Fig; 3. This comprises a high frequency attenuatorI68, a shunt-connected inductance member HST and variable capacitormember I68. The capacity I68 can be varied to produce. the. level andphas adjustment desired.

Reference will now be made to. Fig. 7 which is. a single line schematicdiagram of a control circuit suitable for use in the path C of thecircuit. shown in Fig. 2; or, in other Words, it can be used as thecontrol circuit 59 of. Fig. 2. Be fore describing the circuitarrangement of. Fig. 7 it. seems best to describe its function. Supposethe circuit has been lined up initially to have zero-delay distortion.Suddenly the phases-tot the: N pilot channels are shifted respectivelyby increments c1, ,82, B3, etc. where the subscript ineach case denotesthe number of the channel. It is assumed satisfactory if the automaticdelay distortion compensator so operates that spaced. pilot frequencies,this keeps AB/nw. a. cnstant over the entire transmitted frequencyrange, where A,B=',82-l3r=fl3zfl2, etc, and: Aw=21r13230-=41rF. The useof to represent delay instead of is. assumed satisfactory so long aswiggles in the linecharacteristic have a periodicity considerablygreater than 2F. It is desirable. that changes. introduced by the delayadjuster 5.2 shall be gradual between pilot, frequencies. so that nosharp irregularities are introduced. It: happens that when. any two:frequencies f1. and f2 beat: togather in a demodulator and thedifference. frequency. is: considered, any sudden phase increments. inf1. and f2, such as in and p2, respectively; appear in the differencefrequency as an. increment equal. to flz-fia. In order to provide. atest for B2L-[31,,B3i-B2, p4.fl3,. etc. the N pilot-channels; are.connected to N 1:I demod'ulators as in the arrangement of Fig. '7. Thesedemodulators. are designated. by the. referencev characters I'II', I12,I133, I14, I15, I16}. I11 H191, I88. At: the out:- puts of the:demodul'ators are low-pasefilters I'8I, I 82; I83, I84, I85, I86, I811.I39, I10, andat. the outputs of all. of these filters the differencefrequency, 2F, appears. In each channel, the; phase shift is.respectively equal (within. a constant) to fl2-fi1, iii-B2, 64 63, etc.In order totest these phase increments for equality they are; comparedto some: standard, for example; to. [34'B3 appearing: at the. outputv oflow-pass. filter I83, or in similar fashion, any other filter output canbe selected. An accurate: means of detecting phase shift is the:QIl-degree, push-pull detector shown. in Fig. 8; A phase, detector suchas that. shown in Fig. 8- is sensitive to .01 degree even. when levelchanges; occur in the two inputs of magnitude about one-half decibel.For the frequency interval" 2F=13 230- cycles; the envelope delaysensitivity is .0621 microsecond. In order: to compare phase changes atthe output of low-- pass filter I83: with phase changes in the output:of the other low-pass filters, N -2 phase detectors I91, I92, I-93-,I96; I95, I96 266 are. arranged as shown in Fig. '7, each deriving oneinput from l0w-pass filter I83 and the other input from some otherlow-pass filter (I81, I82, I84, I85, I86. I8'T' I89 or I90). In tandem.with these connections are connected Nl' manual phase and leveladjusters 20-I, 2.02, 203,. 204', 20-5,. 20.6. 207 209', 2I 0 so setthat when no delay distortionexists on the line, the two inputs of eachphase detector are 90 degrees apart, and. thedirect current output is.then zero. (It is; possible to omit thephase adjuster 203 if the othershave sufiicient range.)

The phase detector (I92 for example) shown in- Fig. 8 comprises aresistance: bridge 2I I across the upper and lower corners of which is.appliedone. wave of frequency 2F from transformer 212 connected to onephase and level adjuster (20lto 2m):- of Fig. 7 and across the right.and left corners of which is applied a wave of frequency 2F fromtransformer Zl'3i connected to another one of the phasev and leveladjusters 20;! to 21-0. The upper: and lower corners are. connected re.-spectivelyto rectifiers' 2H8 and 2I5. Between the positive terminals of"these rectifiers is connected: a series: circuit comprising resistors2H5 and ZIT the common terminal 2 I8. of which is. connected. to theright-hand corner ofv the. bridge 2| I. S'eri ally connected. resistorsZIB and 2|! are shunted by a condenser 2.I-9. The operation of circuitslike. that shown. in Fig. 8 is well known and will only be given a briefdescription here.

The current in the upper right arm of the bridge 2I I is the vector sumof the two sinusoidal waves from sources 202 and 203, and the current inthe lower right arm ofthe bridge 2I I- is thevector difference of thesewaves. When the magnitudes of the two inputs 'are -equaL the vector am mbe Pr g dbY-i g j E '-'*2A 60. 1% and the vector diiferenceby tional toplus the voltage of a biasing battery 220. When :90 degrees and athermistor heater or load is connected to leads Hi the current flowingtherein is due entirely to the voltage of battery 220. But if the twoinput voltages are not in QO-degree relationship the load current isincreased or decreased by an amount proportional to (cos -sin Returningagain to the control circuit of Fig. '7, it will be noted that whenl83-fl2 /84fi3, there is a direct current component in the output fromphase detector I92 superimposed upon the current from battery 220, sothat the magnitude and direction of the superimposed component are respectively indications of the magnitude and sign of the inequality ofthe two phase increments. Consequently the resultant direct current inleads 22l serves as an indication of the amount of the required variabledelay for compensation in the frequency region .between Fc-I-F and F+3Fin order to shift phases so that finally fl3;?2=,84 8s. For example,suppose it is desired to increase 53-132. A phase hump similar in shapeto the attenuation hump in Fig. 6 with a peak at FC+3F shown by thedashed curve can be invalues on line. The sendia arrangements shouldbe'stable so that this phase relationship remains fixed within about :01degree in-each case in order not to produce appreciable subse quenterrors in regulation. For-the same reason, the output levels of-thepilot frequencies should remain constant within about $.05 decibel. consequently, temperature control of all filters and phase shiftersis'desirable and also stabilization of the harmonic generator I 4. It isdesirablefto give all "sending circuits the same adjustment throughoutthe plantso'that they are "readily interchangeable. Thereceivingarrangement er Fig. 2 is connectedtothesending arrangement of Fig. 1'through zerolin'e and the --pilot' channels and manual adjustments ofthe receiving arrangement are setso that complementary parts of thedelay and loss adjusters give over-all fiat characteristics. It is alsoimportant for certain parts of the receiving circuit to be kept atconstant temperature and to be as stable as the sending circuits. Afteradjusting a receiving circuit it can be removed to its finaldestination. Each section of circuit to be controlled by the automaticdevice is first given as flat an adjustment of loss and delay aspossible with the basic equalizers of the system before the automaticdevice is added.- Consequently, straightaway loss and. delay distortionmeasurements are desired. The narrow band-pass filters which separatethe pilot channels'from the television current should have highimpedance to prevent unfavorable shunt loss and delay of the throughtransmission.

It is obvious that the amount of apparatus for one sending and onereceiving circuit .as shown in Figs. 1 and 2 is considerable. Referenceis made to the parent application, Serial No. 786,745, for a list ofthis apparatus and for a description of a switching arrangement forreducing the amount of equipment required. Claims to the equalizer shownin Fig. 5 of the present application are contained in a copendserted bysome suitable network. In terms of envelope delay is do! this phasecharacteristic would have a delay hump at about Fc-f-ZF. It is thusevident that the various phase detectors (Isl to 280) are to controldelay humps having peak values about mid-way between the associatedpilot frequencies. Fig. 6 can be assumed to apply to desirable delayequalizers at average conditions if the "humps are taken to denote phaseshift instead of loss. It is obvious that as the device of this sortmakes .B3-- 32=/34/ 3, it also holds etc. The outputs of the phasedetectors l9l to 200 can be used to control thermistors in the delaydistortion equalizer 52 shown in Fig. 5.

In lining up and adjusting the apparatus, the relative phases of thepower channel should be definitely adjusted at the sending end in such amanner as not to cause serious resultant peak ing application, SerialNo. 105,517, filed July 19, 1949.

' Obviously, various changes can be made in the equalizer describedabove without departing from the spirit of the invention.

What is claimed is:

1. An attenuation equalizer which produces substantially no delaydistortion comprising a pair of input terminals, a pair of outputterminals, and a plurality of parallel circuits connected between theinput and the output terminals, respectively, each parellel circuitcomprising a series connection of, in order, a band-pass filter adaptedto pass a frequency band of constant width, but the various frequencybands for the respective filters being different, a delay equalizer tocompensate for any delay produced by the filter, and a resistance, eachfilter having a loss vs. frequency characteristic which is substantiallysinusoidal and is symmetrical around the tuned frequency of the filterand the intensity of the energy transmitted thereby determined by thevalue of said resistance.

2. An attenuation equalizer which produces substantially no delaydistortion comprising a pair of input terminals, a pair of outputterminals, and a plurality of parallel circuits connected between theinput and the output terminals, respectively, each parallel circuitcomprising a series connection of, in order, a band-pass filter adaptedto pass a frequency band of constant width, but the various frequencybands for the respective filters being different, .a delay equalizer tocompensate film any delay produced by thefilter, and a. resistance, eachfilter having a :loss vskfrequency (characteristic which .issubstantially v'sinuso'idai and is symmetrical around the tunedfrequency of the {filter and the intensity ref the energy transmittedthereby determined :by the value of said resistance, and meansternary-mg the values of said resistances in response :to individualcontrol signals.

H3he combination of elements as in claim :2 in which said last-mentionedmeans includes 'a thermistor' tor each 3paral1el circuit.

4. attenuation equalizer of claim 1 :in

which the various bendpess filters are centered 15 REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES 'PATENTS Number Name Date 932,435 Aileen Oct. 31, 19331,961,329

:Beers June 5,1934

